If there isn't room, then don't do it. I cannot stand it when wankers on Harley's try it and end up having to lean the bike way over to avoid a mirror and they almost always look like they couldn't lift it if it fell!!
I repeat.... If there isn't room, then don't split the traffic.
Having said that, if they get to the front and cannot beat me, then pick a lane at least and be gracious in defeat, don't get nasty and try to kick my pride and joy. I will follow you to the ends of the earth and snap you in two!! Rant over....